Chromatographic separation columns are the heart of well-developed analytical systems. The chromatographic column may be a metal or glass tube filled with a solid, inert, particulated support coated with a stationary phase that causes the separation of components or analytes in a test sample or filled with uncoated particles that have surfaces that are chromatographically active. These columns are known as packed columns. The column may also be a long, very small diameter open tube with the interior surface of the tube coated with a stationary phase. These very small diameter open tubes are known as capillary columns. Both types of columns are used in gas chromatography. The active stationary phase in either packed or capillary column gas chromatographs may be a liquid or a solid. If liquid, the procedure is known as gas-liquid chromatography (GLC). If solid, the process is called gas-solid chromatography (GSC). In gas-liquid chromatography, the sample dissolves in the stationary phase. The interaction of the sample component and the stationary phase and the retention time is based on solubility. In gas-solid chromatography, the sample-stationary phase interaction and the retention time is based on adsorption.
The solid stationary phases in gas chromatography are high surface area materials which cause the separation by the interaction of the surface with the sample analytes or compounds. In the packed columns, materials such as silica gel, porous silica, synthetic zeolites or molecular sieves, alumina, activated carbon, graphitized carbon, carbon molecular sieves and styrene divinylbenzene resins have been used. Inorganic materials, such as silica gel, porous silica, molecular sieves, and alumina are activated by heating to drive off water and to make the surfaces active chromatographically, i.e. capable of chromatographic separations. Silica gel can separate lower molecular weight compounds but cannot be used for higher molecular weight compounds because of the long retention time.
Porous silica and silica gel,inorganic non-hybrid solids made specifically for use in a chromatograhic column, are not only used uncoated as stationary phases but also as the support for coatings of chromatographically active liquid layers. The activity of such layers depends on both the liquid phase used and the mount or thickness of the liquid phase used. As thicker coatings are used, the separation becomes more of a gas-liquid separation than a gas-solid separation. The column can be a metal, glass or fused silica tube containing a conventional coated or uncoated solid particle packing material or support. Solid packing materials include uncoated high surface area solids described hereinabove which can separate the components of simple, low molecular weight mixtures. Packed columns for more complex mixtures are made of a solid particle support, such as particles of diatomaceous earth, coated with high molecular weight polymeric stationary phase materials, such as methyl or methylphenyl silicones. Polydimethysiloxane (PDMS) is used extensively as a stationary phase coating in commercially available gas chromatographic columns. Packed columns are relatively easy to make and are relatively inexpensive. As the sample or analyte mixtures become more complex, the packed columns, even those with liquid stationary phases, must be made longer and longer for accurate analytical results. Packed columns of 10, 20 and even 50 feet in length have been used but become more difficult to work with because increased pressure drops make it difficult to maintain required carder gas flow rates. As will become apparent hereinbelow, the present invention provides improved packed columns attributable to the hybrid organic-inorganic solid stationary phases of this invention.
Silicone polymers are extensively employed as liquid stationary phases in gas-liquid chromatography in both packed and capillary columns. Polydimethylsiloxanes (PDMS), for example, are widely used as the stationary phase in the analysis of hydrocarbons, alcohols, aldehydes, esters, ketones and other compounds. Diphenyldimethylpolysiloxanes, methylphenylpolysiloxanes, cyanopropylmethylphenylsiloxanes and trifluoromethylpolysiloxanes are other examples of silicones that are used as liquid stationary phases. Non-silicone organic polymers, such as the various polyglycols, polyesters, high molecular weight hydrocarbons and a variety of other polymeric compounds are also used in analytical procedures where these stationary phases are chromatographically active with the compounds of the test samples. Compounds such as squalane, beta, beta-oxydipropionitrile and dinonyl phthalate are used for special separations.
Capillary columns were developed after packed columns. Because they are more efficient, they have come into broader use, particularly for the more complex mixtures to be analyzed. Even polar compounds could be analyzed in capillary columns, once the technology for deactivating the interior surfaces of fused silica tubes was developed. The stationary phase in a capillary tube is deposited on the inner surface of the tube. Silicones, especially methyl silicones such as the polydimethylsiloxane mentioned above, are used extensively in capillary columns. The stationary phase is generally applied to the inner surface of capillary tubes by the static method. In this method, the stationary phase is dissolved in a volatile solvent. The capillary tube is filled with the solution. One end of the tube is closed off. The solvent is slowly evaporated from the tube by applying a vacuum to the other end of the tube. As the solvent evaporates, the stationary phase is deposited as a uniform film on the inner surface of the tube. The stationary phases of this invention, the hybrid organic-inorganic sol-gels, have been applied to the inside surfaces of tubular supports by this method. In capillary columns with an internal diameter of 250 microns, the film thickness of stationary phases can vary from 0.1 micron to about 1 micron. In larger diameter tubes or columns, such as those with an internal diameter of 530 microns, the stationary film thickness can vary from about 0.5 to about 5 microns. As the film thickness is increased, the retention time of the sample components is increased. At the same time as the film thickness is increased, the efficiency, i.e. the peak sharpness is decreased. In order to allow for the separation of low boiling compounds, various strategies are employed.
Although not analogous to gas chromatography, open tubular or capillary columns for reverse phase high-performance liquid chromatography (HPLC) have been prepared by first developing an inorganic high surface area film on the inside surface of a fused silica tube using a sol-gel process. A layer of an organic stationary phase, for example an octadecyl group, is thereafter bonded or grafted to the high surface area silica derived from the sol-gel using dimethyloctadecylsilane. Such inorganic high surface areas have been disclosed as being prepared by the hydrolytic polycondensation and ulterior gelling of tetraethylorthosilicate (TEOS), also known by the name of tetraethoxysilane, a tetrafunctional silicon alkoxide. Bonding or grafting the organic stationary phase to the silica layer is acknowledged to be difficult. This is disclosed in detail by Crego et al in an article entitled Preparation of Open Tubular Columns for Reversed-Phase High-Performance Liquid Chromatography in Anal. Chem. 1993, 65, 1615-1621. The employment of sol-gel processes in the chromatographic field is believed to have been restricted to making essentially inorganic products. Where chromatographically active organic stationary phases have been used in conjunction with sol-gels, silica gels and porous silica, they have been applied to an already produced inorganic glass material.
Because the early sol-gel processes were primarily devoted to producing glasses or glasslike inorganic products, the organics in the monomeric precursors were primarily directed to making the alkoxides or alkoxysilanes soluble in organic solvents. Even though the organic groups of the alkoxysilanes were essentially removed during hydrolysis, there was a concern that organic residues could cause problems in the polymerization or in the heating to form the glass. More recently, however, the incorporation of organics into inorganic networks has provided composite or hybrid materials with different composite properties on a molecular scale. The properties of the sol-gel derived organic-inorganic materials depend upon the type of monomers used as starting compounds or reactants. New contact lens materials, scratch resistant coatings, oxycarbide ceramics and other organic-inorganic products are made by sol-gel processes employing inorganic network producing monomers such as TEOS, mixed with monomers, oligomers and polymers that provide the modifying organic moiety within or on the inorganic network. There has been a suggestion in the prior art that the proper selection of organic as well as inorganic functions could lead to the use of the gels as they are, e.g. as selective adsorbents or membranes. Mixtures of ethoxysilane and methylethoxysilane monomers have been investigated as sol-gel variations and reported by H. Schmidt in an article entitled New Type of Non-Crystalline Solids Between Inorganic And Organic Materials in the Journal of Non-Crystalline Solids 73(1985) 681-691. Schmidt reports the relative hydrolysis rates of some of these monomers and speculates on the possible polymerisates from different reactions. The introduction of epoxy groups is also discussed, with further speculation that the proper selection of organic as well as inorganic functions may lead to the use of gels, e.g. as selective adsorbents, membranes or in other applications. It is suggested that heat in most cases would result in the destruction of the organic functions.
Efforts to further improve the effectiveness of gas chromatographic columns continue, with considerable efforts concentrated on the stationary phase. There is a need, for example, for columns with unusual resolving ability, particularly columns with low polarity, that is, columns approaching the polarity of squalane, the least polar of any of the heretofore known stationary phases. It is difficult to coat squalane on the interior surface of a capillary column and the squalane has a very limited upper temperature limit. There is also a need to extend the ability of chromatographic columns to resolve lower boiling components. Chromatographic columns increase their ability to separate lower boiling compounds by increasing the surface areas of the stationary phase. This is evident in the earlier efforts to improve the resolving power of the various columns by using a variety of adsorbents with high surface areas including those described above.
In a related field, adsorbent columns or cartridges containing sorbents such as silica gel, alumina, activated carbon or porous polymers have been used to extract and concentrate trace compounds that are present in gases such as air and liquids such as water. The retention characteristics of the sorbents are, of course, important to the ease and efficiency of collecting trace analytes. Trace amounts of analyte pollutants in air, for example, are concentrated in such trap columns to raise the concentration to detectable levels of the available analytical equipment. This may be accomplished by continuously passing air through the column for a time sufficient to retain detectable levels. The heated inert gas of a chromatographic analyzer may be passed through the adsorbent containing the concentrated trapped analyte to elute and separate the retained compounds. The adsorbent column or cartridge is placed in the system before the chromatographic column and heated quickly, allowing the carrier gas to sweep the concentrated analyte compounds into the column, where the compounds are separated and identified. The concentration of the compounds is determined by the signal strength measured by the detector.
It would be desirable and it is an object of this invention to provide stationary phases and adsorbents having a high surface area and sufficient thermal properties using a hybrid organic-inorganic sol-gel produced from alkoxysilanes that produce an inorganic network with a modifying organic moiety within the inorganic network.
It is also an object ot this invention to provide chromatographic columns that are particularly useful in analyzing hydrocarbons and adsorbent columns or cartridges that are particularly useful in concentrating hydrocarbons from streams of gases, such as air or liquids, such as water.